Glaucoma patients and post-operative patients of eye surgery require regular monitoring of the IOP of their eyes in order to diagnose degenerative conditions which may lead to degraded sight and/or blindness without immediate medical treatment. Accordingly such patients must make frequent trips to their ophthalmologist's office for this regular monitoring of their IOP with conventional mechanical impact type tonometers. This becomes a nuisance to the patient after a time leading to patient resistance to compliance. In addition, the only measurement of the patient's IOP that the doctor can use for diagnosis is the pressure that exists at the time of the office visit. Therefore, if the pressure is normal at the time of the visit, but becomes high thereafter, the patient's actual risk of blindness may be misdiagnosed. Also, if the pressure measured at the time of the office visit is high for reasons other than eye degeneration, the patient may be falsely diagnosed and be required to undergo therapy that may not be needed.
Intraocular pressure has been known to fluctuate widely during any given period of time and thus, should be monitored many times during the period of a day in order to gain an average or representative IOP which in turn may be tracked for diagnosis. Attempts have been made to permit glaucoma patients to monitor their IOP at home many time during the period of a day with a self-tonometry portable instrument. Reference is made to the paper “Self-Tonometry to Manage Patients with Glaucoma and Apparently Controlled Intraocular Pressure”, Jacob T. Wilensky et al., published in Arch Ophthalmol, Vol. 105, August 1987 for more details of such a device. This paper describes a portable, tonometer instrument consisting of a pneumatically driven plunger, fitted with an elastic membrane, that slowly comes forward and appianates the cornea. Applanation is detected by an internal optic sensor and the pressure necessary to achieve applanation is registered and displayed automatically. The patient is able to prepare the eye and self-tonometer and activate the instrument for taking the measurement. However, the device proposed is relatively large and bulky, about the size of an attache' case, for example, and not conducive to convenient transport with the patient during normal daily routine in order to measure IOP. In addition, the proposed technique requires special eye preparation by instilling a topical anesthetic in the eye prior to tonometric measurements.
Also, very crude attempts have been made to develop methods of non-invasively monitoring IOP using passive electronic circuitry and radiotelemetry disposed at the eye. In the papers of R. L. Cooper et al. namely, those published in Invest., Ophthalmol Visual Sci., pp. 168–171, February 1977; British JOO, 1979, 63, pp. 799–804; Invest, Ophthalmol Visual Sci., 18, pp. 930–938, September, 1979; and Australian Journal of Ophthalmology 1983, 11, pp. 143–148, a miniature guard ring applanating transsensor (AT) which included electronic components that changed in resonance proportional to the IOP was mounted in an acrylic or sauflon haptic contact lens element that was individually designed for the human eye. The AT was mounted in the lower part of the scleral haptic so that it applanated the inferior sclera under the lower lid. The whole haptic ring was placed in the conjunctival fornix. IOP was monitored from the AT with an automatic continual frequency monitor (ACFM) attached by adhesive and elastic bands to the exterior of the lower eye lid. The ACFM induced in the AT electromagnetic oscillations at varying radio frequencies via a magnetic coupling of inductive coils and monitored for its resonant frequency representative of IOP. This device is clearly uncomfortable and bulky, minimizing expected patient compliance. In addition, the device measures IOP by applanation of the sclera, which is a rather unconventional method of measuring IOP.
In another paper reported in Investigative Ophthalmology Reports, pp. 299–302, April, 1974 by B. G. Gilman, a device is presented for measuring IOP of a rabbit in a continuous manner with strain gauges mounted (embedded) in soft flush fitting, silastic gel (hydrogel) contact lenses. The exact shape of the eye of the rabbit was obtained by a molding procedure. Leads of the strain gauges extended from the lens and were connected to a wheatstone bridge arrangement for measurement taking. The paper suggests that the embedded strain gauges may be used with a miniature telemetry package completely contained in a hydrophilic hydrogel contact lens for continuous, noninvasive, long duration monitoring of IOP, although no design was provided. This device proposes wire connections for telemetry which entails wires to be run out of the eye under the eyelid. Also, the proposed approach requires the molding of a special contact for each individual eye, a practice which would make widespread use unattractive and expensive.
In 1993, an IEEE paper was presented by C. den Besten and P. Bergveld of the University of Twente, The Netherlands, proposing a new instrument for measuring area of applanation entitled “A New Tonometer Based on Application of Micro-Mechanical Sensors”. This new instrument is based on the Mackay-Marg principle of tonometer operation in which a plate having a diameter of 6 mm or less is pressed against and flattens a portion of the cornea of the eye, referred to as “applanation”. In the middle of the plate is a small pressure sensitive area that is pressed against the flattened portion of the cornea with a slowing increasing force while the pressure area is electronically measured. The applanation sensor of this new-instrument comprises a micro-machined plunger and pressure sensing electronics on three electrically insulated levels of a silicon substrate resulting in a modified Mackay-Marg tonometer in which the radius of the flattened area and the distance between the periphery of the applanation and the pressure center can be measured to render a more accurate pressure area measurement. In the work presented in this paper, the researchers did not actually propose a pressure sensor or transducer. In addition, it is not clear if, for as long as the eye is applanated, there is a need to know the area of applanation. Sufficient applanation is usually determined by the difference in trough height from the peak to dip of the pressure profile. The dip is unlikely to occur unless sufficient applanation is achieved.
Also, in the U.S. Pat. No. 5,830,139 entitled “Tonometer System for Measuring Intraocular Pressure by Applanation and/or Indentations”, issued to Abreu on Nov. 3, 1998, a tonometer system is disclosed using a contact device shaped to match the outer surface of the cornea and having a hole through which a movable central piece is slidably disposed for flattening or indenting a portion of the cornea. A magnetic field controls the movement of the central piece against the eye surface to achieve a predetermined amount of applanation. A sophisticated optical arrangement is used to detect when the predetermined amount of applanation has been achieved to measure IOP and a calculation unit determines the intraocular pressure based on the amount of force the contact device must apply against the cornea in order to achieve the predetermined amount of applanation. The magnetic and optical arrangements of this device requires special alignment and calibration techniques rendering it difficult for use as a self-tonometry device.
While the various foregoing described U.S. patent and papers propose various devices and instruments for tonometry, none appears to offer a viable inexpensive, convenient solution to the immediate problem of self-tonometry. The present invention overcomes the drawbacks of the proposed instruments described above to yield a simple, inexpensive and easy to use instrument that completely automates the tonometry process and offers post-processing of tonometer IOP readings from which a proper elevation and diagnosis by an ophthalmologist may be performed.